The Inner Game of Tennis: The Classic Guide to the Mental Side of Peak Performance is not really about tennis. It's about getting your frame of mind right to be able to practice and play at your best, in any sport.
The book draws heavily from the System 1 / System 2 framework that has been discussed before on this site as attributed to Daniel Kahneman. When I first heard that model, it made a lot of sense to me, and so to see it discussed from a sports point of view was pretty interesting.
The book is mostly about different ways to communicate between your two "systems". I've already tried to implement some of these techniques (not with tennis but with squash) and, while I thought some of it was BS while reading the book, I was remarkably pleased with the results. I didn't measure any "before and afters" quantitatively or anything, though, so it may be all in my head.
I highly recommend the book to anyone looking to get better at some athletic endeavour.